How can you hurt from head to toe?

Fibromyalgia (FM or FMS) is characterized by chronic widespread pain and allodynia [pain from stimuli that shouldn't cause pain].
Other symptoms include debilitating fatigue, sleep disturbance, and joint stiffness. Some patients also report difficulty with swallowing, bowel and bladder abnormalities, numbness and tingling, and cognitive dysfunction. Fibromyalgia is frequently [not always] combined with psychiatric conditions such as depression and anxiety and stress-related disorders such as post-traumatic stress disorder. Not all fibromyalgia patients experience all associated symptoms.
 There is evidence that environmental factors and certain genes increase the risk of developing fibromyalgia – these same genes are also associated with other functional somatic syndromes and major depressive disorder.
Research shows the pain control system in the skin, spinal cord, and brain of fibromyalgia patients is overloaded, offering a reason for why you ache all over. In particular, immune cells that generally do not cause pain contribute to the flu-like fibro symptoms that make your whole body hurt.